1. Field of the Invention
The present invention relates to a small-sized, high-power, highly efficient, and highly stable microchip solid-state laser device, and particularly to a semiconductor laser pumped solid-state laser device.
2. Description of the Related Art
Conventionally, a semiconductor laser pumped solid-state laser device can absorb optical energy from a plurality of semiconductor lasers into a solid-state laser medium, followed by converting the energy to a highly focusing laser beam having a uniform electromagnetic wavefront in a solid-state laser resonator. Thus, the very high optical density can be obtained by focusing the laser beam using lenses. As such, this type of laser device has been applied to a variety of common devices and systems, such as measuring light sources for physics and chemistry, as well as processing, e.g., cutting and welding, of various industrial materials.
It is known, however, that since the energy difference between a pumping wavelength of the semiconductor laser and an oscillation wavelength of the solid-state laser turns into heat in the solid-state laser medium, the temperature increase due to the heat causes a change of a refractive index in the solid-state laser medium, as well as distortion or deformation resulting from thermal expansion, which prevents the stable laser oscillation at high-power operation.
Thus, as a configuration for efficiently exhausting heat generated in the solid-state laser medium to the outside of a heat sink or the like, so-called disk-type or microchip-type solid-state laser devices have been developed and have shown effects, in which one surface of a thin laser medium is in direct contact with a heat sink. Particularly, the microchip-type devices may utilize a simple optical system for introducing a pumping light from the semiconductor laser into the solid-state laser medium, which is advantageous in terms of the reduction in size and cost (see Patent Documents 1, 2, and Non-Patent Documents 1-4 below).
Patent Document 1: U.S. Pat. No. 5,553,088
Patent Document 2: U.S. Pat. No. 6,625,193
Non-Patent Document 1: A. Giesen et al., “Scalable Concept for Diode-Pumped High-Power Solid-State Lasers”, Applied Physics B, Vol. 58, pp. 365-372 (1994)
Non-Patent Document 2: T. Dascalu at al., “90 W continuous-wave diode edge-pumped microchip composite Yb:Y3Al5O12 Laser”, Applied Physics Letters, Vol. 83, No. 20, pp. 4086-4088 (2003)
Non-Patent Document 3: H. Yagi et al., “Y3Al5O12 ceramic absorbers for the suppression of parasitic oscillation in high-power Nd:YAG lasers”, Journal of Luminescence Vol. 121, pp. 88-94 (2006)
Non-Patent Document 4: Dietmar Kracht et al., “Core-doped Ceramic Nd:YAG Laser with Sm:YAG Cladding”, Laser Zentrum Hannover, e. V., Hollerithallee 8, D-30419, CThT5 (2007)
Conventionally, in microchip lasers, a pumping light is introduced into a laser medium region (core) provided in the center of the chip through a side face of the chip. A guide region transparent to the pumping light is provided around the core, and the pumping light propagates to the core with total reflection and no loss, and is absorbed therein. By providing an output mirror externally, laser oscillation takes place in the direction perpendicular to the incident direction of the pumping light, i.e., in the thick-wise direction of the chip.
FIG. 1 shows an example of an optical path of parasitic oscillation in a conventional semiconductor laser pumped solid-state laser device.
This figure shows the shape of a typical microchip seen from the optical axis direction of laser oscillation. A circular laser oscillation medium is provided as a center core 101, surrounded by a light guide region 102 for guiding a pumping light 104. A light entrance window 103 for introducing the pumping light 104 is provided on the periphery of the light guide region 102. Since the semiconductor lasers for obtaining high power are usually arrayed with an emitting surface being formed linearly, the light entrance window 103 is also processed into a linear shape for easier entrance of the pumping light. By making the contour of the light guide region 102 into the shape similar to a square, the pumping light 104 can be introduced from as many as four directions. In addition, the end faces of the light entrance window 103 in the thick-wise direction are mirror-polished for introducing the pumping light 104 into the guide region 102 without dispersion.
As shown in FIG. 1, however, there are a number of optical paths (e.g., 105, 106) in the microchip, taken by lights reflected by the faces including the end faces and circulating around. If optical paths pass through the core 101 as the laser oscillation medium having a gain, as with the paths 105 and 106, such a path forms a laser oscillation optical path in the microchip, resulting in the laser oscillation referred to as parasitic oscillation. When the parasitic oscillation occurs within the microchip, pumping energy absorbed in the core circulates around within the microchip as a parasitic oscillation light and turns into heat by being consumed within the microchip, resulting in the substantial decrease in laser output that should to be extracted as a light to the outside of the microchip.
Moreover, as industrial processing lasers, there is a need for high-power, short pulse width, highly stable, and compact semiconductor laser pumped solid-state laser devices.
In view of the circumstances described above, the present invention is directed to provide a compact semiconductor laser pumped solid-state laser device which can suppress undesirable parasitic oscillation in the microchip and efficiently extract energy to the outside.